Vapor blasting nickel plated steel



3,080,643 VAPOR BLASTING NICKEL PLATED STEEL Dean K. Hanink and Edward L. Bolin, Indianapolis, Ind., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Filed Feb. 5, 1958, Ser. No. 713,279 4 Claims. (Cl. 29156.8)

This invention relates to a method of treating surface coatings on metal articles. More particularly this invention relates to a process which can be used for improving the corrosion resistance of turbine wheels having an electroless or immersion deposited nickel coating thereon.

Protective metal coatings have been used as corrosionresistant surface layers on materials which exhibit excessive oxidation at elevated temperatures and, accordingly, nickel coatings have been widely used to protect ferrous metal parts. Coatings applied by immersion nickel plating are useful in a number of applications since this method has certain advantages over other methods of plating. Immersion nickel plating, sometimes referred to as electroless nickel deposition, chiefly provides an extremely uniform plating thickness regardless of the shape of the workpiece; thus a uniform, dense, nonporous plate of high quality can be formed on workpieces of highly complex contour.

Metal coatings, such as those derived from electroless deposited nickel, have been known to be only partially effective in inhibiting the corrosion of dynamically stressed parts. It has now been found that immersion or electroless nickel plate is deposited having an inherent residual tensile stress which greatly reduces the fatigue life of the plate. The inherent tensile stress of this coating limits its effectiveness when it is employed to protect parts which are subjected to extreme dynamic stresses.

In some instances it is desirable to coat gas turbine rotor wheel disks with a thin protective layer of nickel to inhibit corrosion. Gas turbine rotor wheel disks usually have a circumferential flange rim which has axial grooves therein for retaining blade rows on the wheel disk. A more uniform non-porous nickel coating can be applied to structures such as this by electroless nickel deposition than by conventional electrodeposition. Although the former procedure provides much better nickel distribution over the wheel than conventional electrodeposition, severe radial cracking of the plate frequently occurs after the wheel disk is in operation for only a relatively short period of time. The subjacent base metal is thereby exposed and corrosion therefore occurs. We have now found that corrosion of the electroless nickel plated turbine wheels can be inhibited by treating the coated surface with a vapor blast to reduce the cracking of the electroless nickel plate.

Accordingly, among the objects of this invention are the following, singly or in combination: to provide a method of treating metal surfaces to increase the corrosion resistance thereof; to provide a method of inducing a compressive stress on surface coatings having an inherent residual tensile stress; to provide a method of inducing a compressive stress on an electroless deposited nickel surface; to provide a method of making an article having a compressively stressed coating thereon; and to provide an article of manufacture having a compre'ssively stressed surface coating.

Other objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments thereof.

We have discovered that the surface cracking of dynamically stressed electroless nickel plated structural steel parts, such as turbine wheels, can be avoided by the prac- States atet tice of the present invention. In accordance with the invention, the electroless nickel plated surface of a turbine wheel is subjected to a vapor blast carrying a mixture of finely divided abrasive. The vapor blast impinging on the surface coating induces a compressive stress thereon, thereby increasing its elasticity.

Axial fiow compressors for gas turbines involve high compression ratios and necessarily include a large number of stages. As is well known, such compressor rotors are usually built of a series of coaxial wheels which have integral thick rim portions carrying spaced rows of blades on their circumferential peripheries. The wheels also have spaced thin disk portions which serve to tension the remaining portions against the centrifugal stress imposed by rotation. At its end the rotor has especially strong wheels which include projecting shaft portions for journaling. A central tie rod extends through the rotor and interconnects the sturdier end wheels to retain the rotor in assembled relationship.

The end wheel disks, being secured by the central tie rod, are subjected to axial loading as well as radial dynamic stresses. These end wheel disks must therefore be made from a material having suflicient strength to withstand such stresses. A particularly satisfactory material which can be used for the end wheel disks is a structural steel such as that commonly referred to as A181 9310 and which has the following composition:

Such steel wheel disks are ordinarily forged into general shape and subsequently machined to the precise Structure desired. The above type of steel, although having sufiicient strength to be used as an end whee-l disk in a compressor rotor, is subject to corrosion at both room and elevated temperatures. It must therefore be suitably coated to protect it from deleterious corrosion during operation of the compressor. The machined wheel disk is thereafter cleaned and immersed in the electroless nickel bath. The particular method of cleaning is not important to the present invention and any of the conventional methods known in the art can be used. For example, the steel wheel disk can be degreased by means of an alkali cleaner or by suitable solvents, pickled in a hydrochloric acid solution and, if severely rusted or scale, fluxed in a suitable bath such as one containing 32 parts zinc chloride, 8 parts of ammonium chloride, and 60 parts of water, all measurements by weight.

The cleaned part is then immersed in an electroless nickel bath which typically comprises an aqueous solution of a soluble nickel salt and a suitable reducing agent, such as hypophosphite. A particularly suitable type of electroless nickel bath which can be used is disclosed in co-pending United States patent application Serial No. 556,068, now Patent No. 2, 876,116, entitled Chemical Plating Bath and Process, which was filed on December 29, 1955, in the name of H. J. Jerndrzynski, and which is owned by the assignee of the present invention. This application relates to an immersion or electroless nickel plating bath which additionally contains a stabilizing ingredient, such as molybdic acid, added in the anhydride form M00 Typically, a suitable bath would be formed from the addition of the following to one liter of water:

Grams Nickel sulfate (NiSo -6H O) 16.0 Sodium hypophosphite (NaH PO -H O) 18.0 Acetic acid (CH COOH) a 12.6 Molybdic acid (as 85% pure M 0.020

Successful operation of such baths over extended periods of time is facilitated by buffering the bath solution to a pH of about 5.4. Substances such as acetic acid or propionic acid can be used as a buffering agent, amounts from about 2 to about 30 grams per liter being generally sutficient. In baths having excessively high nickel concentration, chelating agents, such as glycine, glycolic acid, ethylene diamine tetraacetic acid and the like can be used to maintain nickel in solution.

The solutions are prepared for operation by dissolving the required amounts of nickel and hypophosphite salts in water, adding the buffer adjusting the pH of the resulting solution with sodium hydroxide. After adjusting the solution to an initial pH of approximately 4.5 to 6.0, the solution is diluted to volume and the inhibitors or stabilizing ingredients dissolved therein. The resulting solution is then heated to an operating temperature of about 150 F. to 210 F. and the workpiece to be plated is immersed therein for a sufiicient length of time to deposit the desired thickness of coating.

Although the method of the present invention is particularly satisfactory for treating electroless nickel plated surfaces, it is understood, of course, that it may be used for inducing a compressive stress on any plated surface which is deposited having an inherent residual tensile stress. By means of this invention articles having such coatings can be utilized to a much greater extent than heretofore possible.

Generally, the length of time the vapor-liquid abrasive stream should be applied depends upon the specific nature of the surface on which it is directed. However, we have found that in most instances, a period of from about fifteen seconds to two minutes is usually sufficient to impart a compressive stress on electroless nickel plated surfaces. Similarly, the pressure employed is variable. We have found that satisfactory results are obtained by aspirating an abrasive-liquid slurry into an air stream which is under a pressure of at least about 40 pounds per square inch. Usually, however, superior results are obtained by aspirating the abrasive-liquid slurry into an air stream under a preferred pressure of approximately 100 pounds per square inch.

The abrasive which is employed is characterized by a very small particle size. A preferred abrasive has a particle size generally within the range of about 200 grit to 1200 grit or, in other words, particles having average diameters of approximately 0.0029 inch to 0.00028 inch. An abrasive having particle sizes substantially in excess of about 200 grit is generally unsuitable especially if employed without being suspended in a liquid and may detrimentally affect the surface against which it is directed. At present we prefer to employ silicon carbide having a particle size of about 280 grit as an abrasive. It will be understood, of course, that other abrasives such as silicon dioxide, quartz, spent catalyst, walnut shells, wheat, etc., having the desired particle size can also be employed.

Although water is preferred as the liquid to be used in the slurry, generally any liquid can be used provided it is not especially corrosive to the surface coating. It is to be understood, of course, that a suitable rust inhibitor can be incorporated into the liquid if necessary to prevent corrosion or rusting of parts exposed to the impinging vapor blast. Compounds such as sodium dichromate are generally suitable as rust inhibitors.

While the ratio of abrasive to liquid can be varied in different applications, we have obtained superior results using a slurry consisting of about 12 to 15 gallons of water and 45 to 50 pounds of the abrasive or generally about 3 to 4 pounds of abrasive per gallon of water. Particularly satisfactory results are obtained if about one ounce of Wetting agent is incorporated in the solution to facilitate wetting of the abrasive. The ratio of air to slurry may also be varied considerably depending upon the application. However, as mentioned above, We have found that excellent results are obtained by aspirating the slurry into an air stream under a pressure of approximately pounds per square inch.

In general, superior results are obtained using the following sequence of steps when treating electroless nickel coatings. A part is immersed in the plating bath for a sufficient duration to provide the desired thickness of nickel coating. It is next washed to remove any of the bath solution adhering to the coated surface and dried. The coated part is then heated for about four hours at a temperature of about 350 F. for the relief of hydrogen embrittlement incurred during the plating operation.

After cooling, the coated surface is subjected to a vapor blast consisting of an air-abrasive-liquid stream formed by aspirating a slurry consisting of a mixture of about 45 to 50 pounds of Carborundum having a particle size of about 280 grit in about 12 to 15 gallons of water having about one ounce of wetting agent. This mixture is di rected onto the nickel plated surface under an air pressure of about 100 pounds per square inch for a time suificient to remove a residual tensile stress of the surface and impart a compressive stress thereon. A vapor blasting for about 15 seconds to about two minutes usually provides satisfactory results on electroless nickel coatings having a thickness of approximately 0.00075 inch to about 0.00125 inch.

The surface thus treated is rinsed with water or other liquid and thereafter dried preferably with clean compressed air. The surface thereof can be readily inspected to determined if the vapor blasting was excessive by applying an acidified copper surface solution to the surface of the plate. A superficial copper deposit will be formed on any exposed surfaces of the subjacent ferrous metal.

The equipment employed in inducing a compressive stress on a nickel plated surface forms no part of the invention and various conventional spray apparatus can be used. The only equipment necessary in addition to conventional spray apparatus which is necessary to practice the invention are a slurry container and means for injecting the slurry into the air stream and projecting it in a finely divided form onto the surface to be treated.

It is to be understood that although this invention has been described in connection with certain specific examples thereof, no limitation is intended thereby except as defined by the appended claims.

We claim: 7

1. The method of making a turbine wheel which comprises applying to a turbine wheel an electroless nickel coating which has a residual tensile stress, and applying to said coating a vapor stream which is under a pressure of approximately 40 pounds per square inch to 100 pounds per square inch and which contains an abrasive that will substantially pass a 200 mesh screen and be substantially retained on a 1200 mesh screen.

2. A method of making a turbine wheel which comprises applying to a turbine wheel an electroless nickel coating which has a residual tensile stress, and applying to said coating a vapor stream which is under a pressure of approximately 40 pounds per square inch to 100 pounds per square inch, said vapor stream containing an abrasive which will substantially pass a 200 mesh screen and be substantially retained on a 1200 mesh screen and which is introduced into said stream in a slurry containing about three pounds to four pounds of abrasive to about one gallon of liquid.

3. A method of making a turbine wheel which comprises applying to a turbine Wheel an electroless nickel coating which has a residual tensile stress, applying to said coating a vapor stream which is under a pressure of approximately 40 pounds per square inch to 100 pounds per square inch and continuing to applying said vapor stream to compressively stress said coating without deleterious erosion, said vapor stream containing an abrasive which will substantially pass a 200 mesh screen and be substantially retained on a 1200 mesh screen and which is introduced into said stream in a slurry containing about three pounds to four pounds of abrasive to about one gallon of liquid.

4. A method of making a turbine wheel which comprises impact working steel into a turbine Wheel, applying 15 to said turbine wheel an electroless nickel coating which has a residual tensile stress, and applying to said coating for about 0.2 minute to 2 minutes a vapor stream which is under a pressure of approximately 40 pounds per square inch to 100 pounds per square inch, said vapor stream containing an abrasive which will substantially pass a 200 mesh screen and be substantially retained on a 1200 mesh screen and which is introduced into said stream in a slurry containing about three pounds to four pounds of abrasive to about one gallon of liquid.

References Cited in the file of this patent UNITED STATES PATENTS 1,985,332 Ward Dec. 25, 1934 2,037,732 Mudge Apr. 21, 1936 2,680,286 Willgoos June 8, 1954 2,795,040 Antel et a1 lune 11, 1957 OTHER REFERENCES Moore: Shot Peening and the Fatigue of Metals, American Foundry Equipment Co., 1945.

Manseil: How Surface Peening Improves Metal Parts," Steel Processing, October 1948,

Wieschhaus: Uses of Shot Peening Other Than for Fatigue Durability, Product Engineering, August 1947.

Shot Peening, American Wheelabrator and Equipment Corp, 1947.

Liquid Honing, Steel Magazine, November 27, 1944, page 100.

Brenner et al.: Part of the Journal of Research of the 20 National Bureau of Standards, Research Paper RP 1835,

vol. 39, November 1947, pages 385-395.

Graham: Electroplating Engineering Handbook; Reinhold Bublishing Corp, 1955, pp. 336-338.

Vapour Blast Liquid Honing, Machinery, March 4,

5 1948, pp. 304, 30s. 

1. THE METHOD OF MAKING A TURBINE WHEEL WHICH COMPRISES APPLYING TO A TURBINE WHEEL AN ELECTROLESS NICKEL COATING WHICH HAS A RESIDUAL TENSILE STRESS, AND APPLYING TO SAID COATING A VAPOR STREAM WHICH IS UNDER A PRESSURE OF APPROXIMATELY 40 POUNDS PER SQUARE INCH TO 100 POUNDS PER SQUARE INCH AND WHICH CONTAINS AN ABRASIVE THAT WILL SUBSTANTIALLY PASS A 200 MESH SCREEN AND BE SUBSTANTIALLY RETAINED ON A 1200 MESH SCREEN. 